The last hop on those Virgin Blue flights was HNL-PAE, so maybe that's what was heard. Come to think of it, if I recall correctly from Flightaware, the Virgin Blue aircraft came over filed as New Zealand tail numbers, and not SWA flight/tail numbers.
FWIW, I rode jumpseat on a BTR-MIA charter some years ago (taking LSU fans to the Orange Bowl) and an EAL flight that heard ATC speaking to us asked what we were doing down there. The captain replied "proving run" and the ATC freq was pretty quiet after that..
As SKC said, there's been not one iota of info on these stealth proving runs within our office...